Newton's law of reptile and amphibian keeping. Mix them not. Never ever mix them. This also goes for your iguana, unless you want to have another iguana, in which you'd need a female and an enormous cage.
They will eat any fish they can fit in their mouth, and will get picked on by bigger fish. They will generally give you a rash if they touch your skin. The links that snakefin posted have some great info! You can make a nice setup for them with anubias in the water area and snake plants on the land area- very low maintenance.
My kids have firebelly toads in a long 20gl. About half the tank has gravel covered with moss, for them. The rest is water with two logs, that rest half on the gravel. There are 6 guppies. They have all been together for over 4 months now, and all seem fine. As long as you keep up on water changes, and filter changes, I think it takes care of any poison issues. ( IMAO )
I think the rash depends on the person... I touch fire bellies all the time and have never had any sort of measurable reaction, most people don't. the toxin is very mild to us but would be more harmful to small would-be predators and fish.
I've held probably.. close to 500 newly imported ones and never had a rash or anything either, but my kids got a rash from just petting them, as have some of the kids who work for my store. They are best left untouched at any rate!